PSN-L Email List Message

Subject: Re: profound instrument differences
From: Brett Nordgren Brett3mr@.............
Date: Sun, 17 Feb 2008 10:26:41 -0500


At 08:56 AM 2/17/2008 -0500, you wrote:
>It appears I haven't been clear (or emphatic) enough in stating the 
>profound differences between a vertical seismometer and a
>pendulum operating as (i) typical horizontal seismometer and/or (ii) 


>     Unlike the vertical that has become standard (influenced greatly by 
> LaCoste), an ordinary pendulum is not capable of simple
>mechanical 'period lengthening' by means of structural rearrangement.  But 
>what makes any pendulum superior to any vertical
>seismometer--is its ability to look at REALLY low frequencies in a way 
>that will ALWAYS be impossible for a vertical.  The
>bottom line is that we need to finally understand that different frequency 
>regimes call for different instruments!
>There never will be a single instrument labeled the SEISMIC-DO-ALL; since 
>the physics refuses to cooperate.
>  Randall


This morning I had an idea which might possibly be an approach to improving 
pendulum performance by using feedback, and I think it would act in a way 
which would meet with your approval.

Start with a 300mm pendulum which is hung from a pivot which can be moved 
horizontally by electronics. ('noisless' motor and leadscrew? or possibly 
black magic might be necessary)  Measure the pendulum angle relative to 
reference vertical (SDC sensor with static plates attached to the moving 
pivot?).  Then apply feedback to move the pivot so as to keep the measured 
angle as close to zero as possible.

Clearly, there are a multitude of practical issues to solve before you'd 
ever have something useful, but I believe that a design based on such a 
concept could possibly have a number of properties which are consistent 
with the performance characteristices you have set out, and which, in 
addition, might provide significantly improved linearity.

I've only thought about this for a few minutes, so please let me know what 
fundamental errors you see.



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